Mineral insulated sheathed assembly with insulation resistance indicator

ABSTRACT

An assembly includes an electrical conductor disposed within an elongate mineral insulated conductive sheath. The electrical conductor is electrically grounded to the conductive sheath. The assembly also includes a test conductor disposed within and electrically isolated from the sheath to provide an indication of the insulation resistance of the assembly.

TECHNICAL FIELD

The present invention relates generally to mineral insulated sheathedassemblies, such as temperature sensing assemblies and heating elementassemblies, and, more particularly, to a mineral insulated sheathedassembly having a conductive element and an insulation resistanceindicator contained within a conductive sheath.

BACKGROUND

A variety of temperature sensors can be used in environments thatrequire the temperature sensor to be contained within a protectivesheath. For example, the temperature sensors can be used inapplications, such as high temperature and/or high pressure processes,that require that the sensor be protected to some degree from theextreme environment. In some applications, the protective sheath is madeof a conductive material that is electrically grounded in the setup inwhich the temperature sensor is deployed. In many setups, thetemperature sensor is configured as a thermocouple with a junction pointthat also is electrically grounded. For example, the thermocouple can beelectrically grounded by electrically coupling the thermocouple to theconductive sheath. The sheath is filled with an electrically insulativematerial to isolate the conductors making up the thermocouple from eachother and from the inner wall of the sheath (except for the junctionpoint). Because the junction point is grounded to the sheath, a measureof the insulation resistance of the temperature sensor (i.e., a measureof the integrity of the electrically insulative material isolating theconductors from each other and from the inner wall of the sheath) cannotbe made. Accordingly, an imminent failure of the temperature sensor maygo undetected until the sensor actually fails. Inaccuracies intemperature measurements also may go undetected.

Assemblies also are used that include heating elements contained withina mineral insulated conductive sheath. In such assemblies, current isapplied to a conductive element within the sheath to generate heat. Thesheath may then be positioned adjacent or wrapped around anotherstructure to keep that structure warm. These assemblies also can fail ifthe integrity of the insulative material in the sheath is compromised.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain embodiments of the invention will hereafter be described withreference to the accompanying drawings, wherein like reference numeralsdenote like elements. It should be understood, however, that theaccompanying drawings illustrate only the various implementationsdescribed herein and are not meant to limit the scope of varioustechnologies described herein. The drawings are as follows:

FIG. 1 is a schematic representation of a cross-section of a temperaturesensing assembly according to an embodiment.

FIG. 2 is a cross-section taken along the line 2-2 of FIG. 1.

FIG. 3 illustrates the temperature sensing assembly of FIG. 1 connectedto an exemplary terminal box according to an embodiment.

FIG. 4 is a schematic representation of a face of the terminal boxshowing the connection terminals according to an embodiment.

FIG. 5 is a diagram illustrating an exemplary procedure for assemblingand testing insulation resistance of a temperature sensing deviceaccording to an embodiment.

FIG. 6 is a diagram illustrating an exemplary procedure for deploying atemperature sensing device and testing insulation resistance during useaccording to an embodiment.

DETAILED DESCRIPTION

Embodiments of the invention provide a temperature sensing assemblydisposed within a conductive sheath. The temperature sensing assemblyincludes a thermocouple with a grounded junction point and anelectrically insulative material surrounding the conductors disposedwithin the sheath that make up the thermocouple. The assembly furtherincludes an electrical conductor contained within the sheath andarranged to provide an indication of the insulation resistance of theassembly, as will be described in further detail below.

An example of a sheathed temperature sensing assembly 100 according toone embodiment is shown schematically in the cross-section view ofFIG. 1. The assembly 100 includes a sheath 102 made of a conductivematerial (e.g., stainless steel, Inconel, nickel alloy) and athermocouple 104 made of a pair of electrical conductors 106, 108 ofdissimilar materials, such as iron/constantin, chromel/alumel,copper/constantan, joined at a temperature sensing junction 110. Thesheath 102 comprises an open interior into which the pair of electricalconductors 106, 108 extend through an open end 112 to a distal end 114.In the embodiment of FIG. 1, the electrical conductors 106, 108 extendalong the length of the sheath 102 and are joined to form the sensingjunction 110 at the distal end 114 of the sheath 102. The junction 110is electrically connected to the sheath 102, such as by a weld 113, sothat, in use, both the junction 110 and the sheath 102 are at the sameelectrical potential. Generally, in use, the junction 110 and the sheath102 are electrically grounded.

Although FIG. 1 shows the junction 110 at the distal end 114 of thesheath 102, it should be understood that the junction 110 may be locatedat any desired position along the length of the sheath 102. Further,although only one conductor pair 106, 108 is shown, more than oneconductor pair can be included within the sheath 102. The assembly ofFIG. 1 also includes a third electrical conductor 116, referred to as atest conductor, extending into the open interior of the sheath 102through the open end 112 approximately to the distal end 114.

An insulation material 118, such as an electrical insulation material,is disposed about the individual conductors 106, 108, 116 containedwithin the sheath 102. The insulation material 118 generally fills theinterior about each of the conductors 106, 108 of the conductor pair andthe test conductor 116. Although various electrical insulation materialsmay be used, exemplary materials are magnesium oxide (MgO) and aluminaoxide. The insulation material 118 electrically isolates each of theconductors 106, 108, 116 from the others and from the inside wall of thesheath 102 except at the junction point 110. Maintaining the electricalisolation between conductors 106, 108, 116 and the sheath 102 (except atthe junction point 110) helps ensure that the temperature measurementprovided by the junction point 110 is accurate, reliable and will notdrift.

FIG. 2 shows a cross section of the temperature sensing assembly 100taken generally along the line 2-2 of FIG. 1, showing the conductor pair106, 108 and the test conductor 116 disposed within the interior space112 of the sheath 102. The conductor pair 106, 108 and the testconductor 116 are surrounded by the insulative material 118.

In the embodiments shown, an insulation resistance test to measure theintegrity of the insulation material 118 is performed, such as inaccordance with the requirements of ASTM E585 and E780. In general, aninsulation resistance on the order of 1 Gohm at 500 VDC at ambienttemperature is acceptable to ensure the integrity of the temperaturemeasurement provided by the junction point 110. Lower insulationresistances (e.g., in the tens of Kohm range) can indicate the presenceof moisture within the interior space 112 of the sheath 102, which notonly will affect the measurement, but can lead to corrosion of theconductors 106, 108 and ultimately failure of the temperature sensingassembly 100. Thus, the ability to measure insulation resistance at thetime of manufacture and during use can provide useful information.Generally, the insulation resistance would be measured by applying anelectrical potential between the conductor pair 106, 108 and the sheath102. However, for temperature sensing assemblies having the junction 110electrically connected to the sheath 102, a measurement of insulationresistance cannot be made. Thus, in the embodiment shown in FIG. 1, thethird conductor 116 (i.e., the test conductor) is provided to measurethe insulation resistance of the temperature sensing assembly 100.

When deployed in the application in which the temperature measurementsare made, the conductors 106, 108, 116 within the sheath 102 areconnected to a terminal box 120 or other suitable arrangement thatprovides ready access to apply and/or measure electrical signals presenton the conductors 106, 108, 116. As shown in FIG. 3, an exemplaryterminal box 120 with an access cover 121 can be coupled to the sheath102 through an appropriate mechanical coupling 122 and each of theconductors 106, 108, 116 extends from the open end 112 of the sheath 102and is connected to terminals 126, 128, 130 that are accessible to anoperator of the temperature sensing assembly 100. The face of anexemplary terminal box 120 is shown schematically in FIG. 4, whichincludes the pair of terminals 126, 128 electrically connected to theconductor pair 106, 108 of the thermocouple, a third terminal 132 thatis electrically grounded, and a fourth terminal 130 that is electricallyconnected to the test conductor 116. The terminal 132 is electricallygrounded via a conductor 109 that can be directly connected to sheath102 or can be electrically connected to the sheath 102, such as by aconnection to an electrically conductive transition housing. Theinsulation resistance measurement can be made by applying a DC voltageacross the fourth and third terminals 130, 132.

The measurement can be made at various points during themanufacturing/assembly process and before the temperature sensingassembly 100 is deployed to the field. The measurement also can be madeperiodically during use of the temperature sensing assembly 100 in thefield to check the integrity of the temperature measurements and/or todetermine or predict whether a failure has or will occur.

For example, as shown in the flow diagram in FIG. 5, the thermocouple104 and test conductor 116 can be assembled in a mineral insulatedsheath 102 with the junction point 110 welded to the sheath 102 at thedesired temperature sensing location and the sheath 102 can be compacted(block 502). A DC voltage (e.g., 500 VDC or other value appropriate forthe particular assembly 100) can then be applied between the testconductor 116 (block 504), and the sheath 102 and the insulationresistance determined from an electrical measurement made between theconductor 116 and sheath 102 (block 506). For example, the insulationresistance can be determined by measuring the electrical current flowingbetween test conductor 116 and the sheath 102 using appropriateinstrumentation. If the determined insulation resistance is above apredetermined threshold (e.g., 10 Gohms or other value appropriate forthe particular assembly 100) (block 508), then the integrity of theassembly 100 can be deemed adequate and the assembly 100 can be acceptedfor further processing and/or deployment for use (block 510). Otherwise,the assembly 100 can be rejected and discarded or reworked (block 512).

If the temperature assembly 100 is accepted, then further assembly stepsand/or deployment in the field can be performed. For example, as shownin the flow diagram of FIG. 6, an accepted temperature sensing assembly100 can be connected to a terminal box, such as the exemplary terminalbox 120, via appropriate mechanical couplings and electrical connections(block 602). At this point, the insulation resistance can again betested (or might be tested for the first time) by applying a DC voltage(e.g., 500 VDC or other appropriate value) between the test conductor116 and the sheath 102 (block 604). As an example, the voltage can beapplied across the appropriate terminals of the terminal box 120. Theinsulation resistance of the assembly 100 can then be determined basedon measurement of an electrical parameter (e.g., current) between theterminals of the terminal box 120 (block 606). If the determinedinsulation resistance is below a threshold amount (e.g., 1 Gohm) (block608), then, if the assembly 100 has been deployed in the field, itshould be replaced (block 610). Otherwise, the assembly 100 can be usedor continue to be used to monitor temperature in the field deployment(block 612). While in the field, periodic measurements to determineinsulation resistance can be made (block 614) to ensure the continuedintegrity of the assembly 100 and the temperature measurements obtainedtherefrom. If at some point during the life of the assembly 100 thedetermined insulation resistance falls below an acceptable threshold,then the assembly 100 can be replaced (block 610) before it failsaltogether.

Various processes may be used to form the temperature sensing assembly100. One exemplary methodology comprises extending the conductors 106,108, 116 into the interior of the sheath 102, welding the dissimilarconducting materials 106, 108 together at a junction point 110, andwelding the junction point 110 to the sheath 102 at a desired location.The insulation 118 can initially be placed within the sheath 102 in theform of beads. The sheath 102 and insulation 118 can then be compacted(e.g., by drawing, swaging, etc.) 102 so that the insulation 118 fillsthe interstices between conductors 106, 108, 116. At this point in theassembly, the insulation resistance can be measured by applying a DCvoltage (e.g., 500 VDC) between the sheath 102 and the test conductor116 as discussed above. The conductors 106, 108, 116 of the temperaturesensing assembly 100 can then be electrically coupled to appropriateterminals in the terminal box 120 and used to monitor temperature in thefield.

In other embodiments of the invention, the assembly 100 can be a heatercable and one or more of the conductors 106, 108 may be configured asheating elements, where the length and the resistance of conductors 106and/or 108 are selected to provide a desired Watts per foot for theparticular application in which the heater cable is employed. Insulationresistance of the heater cable assembly then can be measured using thetest conductor 116 in the manner discussed above.

While the invention has been disclosed with respect to a limited numberof embodiments, those skilled in the art, having the benefit of thisdisclosure, will appreciate numerous modifications and variationstherefrom. For example, the configurations and techniques describedherein can be applied to test and measure the insulation resistance ofany type of assembly in which one or more conductors are containedwithin a conductive sheath that is filled with an electricallyinsulative material. It is intended that the appended claims cover suchmodifications and variations as fall within the true spirit and scope ofthe invention.

What is claimed is: 1-6. (canceled)
 1. A method of assembling a temperature sensing device, comprising: providing an electrically conductive sheath; disposing a pair of dissimilar conductors within an interior space of the conductive sheath; joining ends of the dissimilar conductors to form a temperature sensing point; electrically connecting the temperature sensing point to the conductive sheath; disposing a test conductor within the interior space of the conductive sheath adjacent the pair of dissimilar conductors; disposing an insulative material within the interior space of the conductive sheath about the pair of dissimilar conductors and the test conductor; applying a voltage between the test conductor and the conductive sheath; and determining an insulation resistance of the temperature sensing device based on the applied voltage.
 8. The method as recited in claim 7, wherein if the determined insulation resistance is above a predetermined threshold, deploying the temperature sensing device to measure temperature.
 9. The method as recited in claim 7, wherein if the determined insulation resistance is below a predetermined threshold, discarding or reworking the temperature sensing device.
 10. The method as recited in claim 7, further comprising compacting the conductive sheath and then applying the voltage between the test conductor and the conductive sheath and then determining the insulation resistance based on the applied voltage.
 11. The method as recited in claim 10, wherein if the determined insulation resistance is above a predetermined threshold, then, coupling the temperature sensing device to a terminal box having terminals electrically connected to respective free ends of the pair of dissimilar conductors, a free end of the test conductor, and the conductive sheath.
 12. A method of testing insulation resistance of a temperature sensing device, comprising: providing a temperature sensing device comprising: an elongate conductive sheath; a pair of dissimilar conductors extending within the elongate conductive sheath and joined to form a temperature sensing point, the temperature sensing point electrically connected to the elongate conductive sheath; a test conductor extending within the elongate conductive sheath and adjacent to the pair of dissimilar conductors; and an insulative material disposed within the elongate conductive sheath about the pair of dissimilar conductors and the test conductor; applying a voltage between the test conductor and the conductive sheath; and determining an insulation resistance of the temperature sensing device based on the applied voltage.
 13. The method as recited in claim 12, further comprising: measuring an electrical parameter between free ends of the pair of dissimilar conductors; and determining temperature at the temperature sensing point based on the measured electrical parameter.
 14. The method as recited in claim 12, further comprising: coupling the temperature sensing device to a terminal box; connecting free ends of the pair of dissimilar conductors to respective first and second terminals in the terminal box; connecting a free end of the test conductor to a third terminal in the terminal box; and electrically connecting the sheath to a fourth terminal in the terminal box.
 15. The method as recited in claim 14, further comprising: measuring a first electrical parameter between the first and second terminals; determining the temperature at the temperature sensing point based on the measured first electrical parameter; applying a voltage between the third and fourth terminals; measuring a second electrical parameter between the third and fourth terminals; and determining an insulation resistance of the temperature sensing device based on the measured second electrical parameter.
 16. A method of testing insulation resistance of an assembly, comprising: providing an assembly comprising: an elongate conductive sheath; an electrical conductor extending within the elongate conductive sheath; a test conductor extending within the elongate conductive sheath and adjacent to the electrical conductor; and an insulative material disposed within the elongate conductive sheath about the electrical conductor and the test conductor; applying a voltage between the test conductor and the conductive sheath; and determining an insulation resistance of the assembly based on the applied voltage.
 17. The method as recited in claim 16, further comprising deploying the assembly proximate a structure to sense temperature at a desired location, and sensing temperature at the desired location.
 18. The method as recited in claim 16, further comprising deploying the assembly proximate a structure to provide heat, and applying a current to the electrical conductor to heat the structure. 